Photoinduced\nReductive C–O Couplings from Unsymmetrical\nBis-Cyclometalated Pt(IV) Dicarboxylato Complexes

Unsymmetrical bis-cyclometalated dicarboxylato complexes\n(<i>OC</i>-6-32)-[Pt(tpy)<sub>2</sub>(O<sub>2</sub>CR)<sub>2</sub>] [tpy = cyclometalated 2-(<i>p</i>-tolyl)pyridine,\nR = <i>t</i>-Bu (<b>1</b>), Me (<b>2</b>), Ph\n(<b>3</b>), CF<sub>3</sub> (<b>4</b>)], are obtained from\nthe reaction\nof <i>cis</i>-[Pt(tpy)<sub>2</sub>] with the appropriate\nPhI(O<sub>2</sub>CR)<sub>2</sub> reagent. Treatment of complexes of\nthis type with different carboxylates (R’CO<sub>2</sub><sup>–</sup>) results in the formation of mixed-carboxylato derivatives,\nnamely (<i>OC</i>-6-43)-[Pt(tpy)<sub>2</sub>(O<sub>2</sub>CMe)(O<sub>2</sub>CR′)] [R′ = <i>t</i>-Bu\n(<b>5</b>), CF<sub>3</sub> (<b>6</b>), Ph (<b>7</b>)], (<i>OC</i>-6-34)-[Pt(tpy)<sub>2</sub>(O<sub>2</sub>CCF<sub>3</sub>)(O<sub>2</sub>CR′)] [R′ = <i>t</i>-Bu (<b>8</b>), Me (<b>9</b>), Ph (<b>10</b>)],\nand (<i>OC</i>-6-34)-[Pt(tpy)<sub>2</sub>(O<sub>2</sub>C-<i>t</i>-Bu)(O<sub>2</sub>CMe)] (<b>11</b>). Irradiation\nof <b>1</b>–<b>3</b> and <b>5</b>–<b>11</b> with UV light (365 nm) in MeCN gives 5-methyl-2-(2-pyridyl)phenyl\npivalate (<b>12</b>), 5-methyl-2-(2-pyridyl)phenyl acetate (<b>13</b>) or 5-methyl-2-(2-pyridyl)phenyl benzoate (<b>14</b>) as the major photoproduct from most complexes, resulting from a\nreductive C–O coupling between a tpy ligand and a carboxylato\nligand. Cyclometalation of <b>12</b>–<b>14</b> at\nthe ensuing Pt(II) species to produce <i>cis</i>-[Pt(tpy)(tpyO<sub>2</sub>CR/R′)], reduction to <i>cis</i>-[Pt(tpy)<sub>2</sub>] and isomerization to (<i>OC</i>-6-33)-[Pt(tpy)<sub>2</sub>(O<sub>2</sub>CR/R′)<sub>2</sub>] are identified as\nsecondary processes in most cases. In contrast, complex <b>4</b> exclusively photoisomerizes to (<i>OC</i>-6-33)-[Pt(tpy)<sub>2</sub>(O<sub>2</sub>CCF<sub>3</sub>)<sub>2</sub>] (<b>4′</b>). The C–O couplings are favored for the most electron-rich\ncarboxylato ligands and occur predominantly from the carboxylato trans\nto N. Consistent with this, a computational study reveals that the\nlowest singlet and triplet LMCT excited states result from electronic\ntransitions to a dσ* orbital distributed along the N–Pt–O\naxis, which would trigger the observed processes.